THREE YEARS AGO, when Congress passed the Cuban Democracy Act to tighten the U.S. embargo against Fidel Castro, congressional sponsors promised it would bring the dictator to his knees within weeks.

This week, after Castro shot down two light aircraft flown in or near Cuban waters by anti-Castro Cuban exiles, those same sponsors were proclaiming that this time they would really do it: The suddenly revived Cuban "Libertad" bill, sponsored last year by Senator Jesse Helms and Representative Dan Burton, "Is the last nail in his (Fidel's) coffin," boasted Burton, while Helms chimed in with a jubilant "Adios, Fidel." We'll see.

Castro's legendary staying power has survived 35 years of the most punitive sanctions ever imposed on another country by the United States. And according to many Cuba specialists, Castro remains firmly in power, with the full support of the only element that could conceivably overthrow him, the army and security forces.

What's more, Cuba's incipient economic recovery, due to large-scale foreign investment and a flowering of private small enterprises, has reportedly begun to replace the despair of a few years ago with a new sense of popular optimism. In other words, at a time when the long failure of U.S. sanctions is more apparent than ever, Congress and a politically pressured White House is lashing out at Castro's cowardly muscle-flexing by codifying into law a policy that is likely to do more harm to legitimate U.S. interests than to Castro.

Clinton's sudden and unconvincing reversal of opposition to the bill might possibly help him carry Florida in the November election, but only at the risk of setting back long-term efforts to move Cuba toward democracy.

Certainly, a forceful U.S. reaction to last week's outrage was necessary, and the steps ordered by the president earlier this week -- closing of charter air routes, restricting the travel of Cuban diplomats and expanding Radio Marti -- constituted an appropriate response.

But in contrast to those measures, passage of the Helms-Burton bill is a step backward. By sealing the sanctions policy into law it deprives Clinton or any future president of the flexibility needed to quickly tighten or loosen the sanctions in reward or punishment of Cuban behavior.

In granting Cuban Americans the right to sue in U.S. courts any foreign investors in nationalized Cuban property -- primarily Canadians and Europeans -- it constitutes a slap in the face to the very allies the United States is trying to persuade to support a global sanctions resolution through the U.N. And in prohibiting the importation of sugars, syrups or molasses from countries that import such products from Cuba, it directly violates a slew of international trade agreements, from GATT to NAFTA.

All this might be acceptable if, indeed, the law hastened a peaceful transition to democracy in Cuba. But 35 years of experience suggests that it will not, although it might hasten a violent popular upheaval that would send massive flows of refugees to U.S. shores. The Helms-Burton law is a case of revenge-driven American foreign policy at its worst.